We have identified and characterized a family of interspersed repetitive sequences from the mouse genome. Individual members of the family (designated MIR-1) have been isolated from mouse genomic libraries. The repeats are about 400 base pairs long and amount to 1-2% of mouse genomic DNA. The sequences represent a group of homologous but non-identical units, and individual members of the family show considerable divergence from one another. The spatial relationships between members of the family and a number of other identified mouse sequences including structural genes have been determined; these sequences are found on the 5' as well as 3' sides of genes at distances ranging from less than 1 to 5 kilobases. The sequences are present in the DNA of all species of Mus. Related sequences are present in the rat genome at a repetition frequency similar to that in the mouse genome.